Which Year?

The prefix is DGB, how to know exactly which year it's from?

You just go by the signatures and see they were appointed.

Robert Alan ‘Bob’ Johnston - tenure Aug 1982 - Jul 1989

John Owen Stone - Jan 1979 - Sep 1984.

You can put it down as nd1982, or even nd1983, whichever year you like. 😁

@ahkai is correct that you can't be certain of the exact year, but I disagree with the year options that were proposed.

If you look at the prefix options in the year line comments, you can see that ‘R78F - P#42d - Johnston & Stone (First prefix DGJ)’ is given as ‘ND (1982-1984)’. So this first prefix is clearly after the OP's prefix of DGB, so we should be looking at a different year line. My suggestion would be ‘R77L - P#42c - Knight & Stone (Last prefix DGH)’ which is given as ‘ND (1979-1981)’.

According to my old Renniks Australian's catalogue, this note is listed as 1982 with the last prefix of DPS. Nothing is mentioned for the first prefix.

My other old Greg McDonald catalogue has this also listed as 1982. Again it also mentioned the last prefix as DPS but not the first prefix.

Listing the year based on the year the Governor was appointed is quite acceptable, imo.